Download The Grenville Province

The Grenville Province
Sudbury
Georgian
Bay
Lake Huron
50
100km
The Grenville Province, a jigsaw puzzle of terranes, was pieced together over more them 500 million years
nug Harbour, Pointe au Baril, Port Loring—this
scenic area of Ontario's cottage country was beneath
a huge mountain range about 1.18 billion years ago.
The towns of Tweed and Apsley were once sites of
powerful volcanoes.
That stretch of dark green rocks from Parry Sound to
just east of Huntsville was formed about 1.45 billion
years ago in another part of the world.
These gneisses near Minden are part of the
Grenville's largest terrane - the
Algonquin
About 1.24 billion years ago, shallow tropical seas
washed over the towns of Westport, Gananoque and
Portland.
This is the Grenville Province of central and eastern
Ontario, a patchwork of many different pieces of
crust, or terranes. These tenanes collided in a pow­
erful mountain building event which thrust up a
major mountain range.
Today, the ancient mountain range has been eroded
by wind, rain and ice. Rocks that were once deeply
buried within the Earth's crust are now uncovered as
the rocks of the Grenville Province.
Glacicd activity stripped the soil from parts of the Grenville
Province
The Grenville Province: Up Close
The making of the Grenville Province is the third
stage in the formation of Ontario. Built from about
1.76 billion to about 1.00 billion years ago, it is the
youngest part of the Canadian Shield.
The rocks of the Grenville Province were baked,
squeezed, stretched and twisted into metamoiphic
rocks by a series of powerful and long mountainbuilding e v e n t s between
billion and
1.00 billion years ago.
T h e Grenville's T w o Belts
The Grenville Province in Ontario is divided into
two major sections: the Central Gneiss Belt and the
Central Metasedimentary Belt, each with a unique
geological history.
Migmatite, a rock with mixed layers of igneous and
metamorphic rock, is common in the Nipissing Terrane
Each of these belts, in turn, is subdivided into
smaller units called terranes.
"Suspect" Terranes
Terranes which are thought to have travelled along
the Earth's plate tectonic conveyer belt are called
suspect terranes.
There is one suspect terrane in the Central Gneiss
Belt—the Parry Sound Ten-ane. Geologists think it
was part of an ancient volcanic island are which
formed at some unknown location thousands of
kilometres away from Ontario.
All the terranes in the Central Metasedimentary
Belt are suspect terranes.
Geologists are still unravelling the complex
Grenville story. Here's a look at what they have
found so far.
The Central Gneiss Belt Terranes
The Central Gneiss Belt is the oldest part of the
Grenville Province. Its oldest terrane is the Nipissing
Terrane, which is 2.73 to 1.45 billion years old.
The Nipissing Terrane is a block of sedimentary
and igneous rocks which formed the south margin
of the Archean Superior Province. These rocks
were folded and metamorphosed, and intruded by
Grenville Province batholiths as a result of the
eol I i sion of Grenville Province terranes to the south
with the Archean rocks.
The rocks of the Central Gneiss Belt have been captured in
paintings of Group of Seven Artists like "A September Gale,
Georgian Bay", by Arthur Lismer
Banded gneiss like this is so abundant in the northern half of the Grenville Province it gives the "Central Gneiss Belt" its
name
Easily recognizable sedimentary rocks, like this conglomerate between Madoc and Renfrew, are preserved in parts of the
Centred Metasedimentary' Belt
The youngest is the Parry Sound Terrane. Most of its
rocks are between 1.45 and 1.35 bilHon years old. In
the middle are the rocks of the Algonquin and Tomiko
terranes. They are between 1.68 billion and 1.45 bilhon
years old.
Central Gneiss Beh Rocks
This northern piece of the Grenville Province is a vast
sea of gneiss, cut by granite plutons. The gneiss formed
when older rocks were deeply buried during mountain
building activity. The resulting heat and pressure caused
the rocks to change, or metamorphose, into gneiss.
The granite plutons of the Central Gneiss Belt were
i ntruded about 1.45 billion years ago. One of the largest
is the Algonquin Batholith. It formed about 30 kilome­
tres below the Earth's surface when red hot magma
flowed up from the mantle. Today, outcrops of the
Algonquin Batholith make up the scenery of the
Algonquin uplands.
Scenes like this view of Kamaniskeg Lake northeast of
Bancroft are typical of the area of the Algoi^qiiin Batholith
A Window for Geologists
The Central Gneiss Belt is a window into the deep crust
for geologists. Its gneisses—once buried to a depth of
up to 20 or 30 kilometres—^allow geologists to com­
pare the shape of batholiths, folds and faults deep
within the crust to those higher up in the crust. Nor­
mally, geologists must rely on images of the deep crust
generated by studies of sound waves passing through
the Earth's crust, like images made using ultrasound in
medicine.
The Central M e t a s e d i m e n t a r y Belt Terranes
The Central Metasedimentary Belt is the largest sec­
tion of the Grenville Province. It is divided into two
sections. The Superterrane and the Frontenac Terrane.
The Superterrane
The large Superterrane is actually four formerly sepa­
rate fault-bounded terranes, each a mix of distinct
sedimentary and volcanic rocks which formed be­
tween 1.30 billion and 1.25 billion years ago. They
were metamorphosed between 1.25 billion and 1.24
billion years ago, during the process of colliding to
form one large supertciTane. At the same time, they
were invaded by plutons of granite.
The Frontenac Ten-ane
This small terrane has plenty of marble, quartzite and
gneiss, but no volcanic rocks. It formed about 1.28
billion years ago.
In parts of the Frontenac Terrane, stromatolites have not
been destroyed by metamorphism
The Central Metasedimentary Bell Rocks
The making of the Central Metasedimentary Belt is a
story of erupdng volcanoes, island arcs, the erosion of
sediments and mountain building. These are the same
processes that made the Superior Province, but there are
some important differences.
The volcanoes in the Central Metasedimentary Belt
fonned in a tropical environment, and were closely
associated with carbonate sediments such as limestone.
The volcanoes were far from the young Ontario—likely
hundreds of kilometres away. Present-day Indonesia
may be similar to the ancient Central Metasedimentary
Belt.
Pillows preserved in volcanic rocks of the Central Metasedimenlcuy
Belt near Bon Echo Park tell us that the lava erupted under water
Getting It All Together
The events which brought the rocks of the Central Gneiss
Belt and the Central Metasedimentary Belt together
were as big, and complex, as any ever seen on Earth.
This makes the geology of the Grenville Province ex­
tremely difficult to decipher.
The clues geologists have been able to read in the rock so
far have told us that the Central Gneiss Belt was partially
assembled by about 1.68 billion years ago, and mostly
together by about 1.45 billion years ago.
Meanwhile, in the Central Metasedimentary Belt, the
four sedimentary-volcanic terranes came together to
make up the superterrane by about 1.25 billion years ago.
Then, the final stages in the assembly of the Grenville
Province took place. Between 1.18 billion years and
1.16 billion years ago the Central Metasedimentary Belt
was completed when the Frontenac Tcn'anc joined the
superterrane, and the Central Gneiss Belt was coinpleted
when the Parry Sound Terrane was pushed over the
Algonquin Terrane.
Finding Volcanic Rocks
Highway 62, between Bancroft and
Madoc, offers excellent viewing of
many volcanic rocks, including
andesite, rhyolite and pillow lava.
Highway 41, near Cloyne, has much
better examples exposed. Pillow lava is
found in many other tocafions
throughout the Superterrane, especially
near the oid volcanic centres. Broken
pieces of pillow lava, called "pillow
breccia" are also common.
At about the same time, these two major belts were
shoved into each other by immense plate tectonic forces
pu.shing northward. With the Canadian Shield for a
backstop, the rocks of the Grenville Province slowly
crumpled, and a range of mountains to rival the Himala­
yas was formed.
Geological activity in the Grenville Province since then
has been much less dramatic. Several tens of kilometres
of uplift and erosion have taken place, with the result that
the mountains have been eroded flat. This took several
hundred million years, but by 570 million years ago the
area was ready for the next stage in its geological history.
Tecfoniie is a type of fine-grained gneiss that
lies along old fault zones which separate the
terranes of the Central Gneiss Bell. Highway
69 in the Parry Sound area cuts through a
tectonite zone dividing the Parry Sound and
Algonquin Terranes.
Tectonites are called "flaggy" rocks. Their
many fine layers can easily he broken
into slabs, which are used as flagstone.
These rocks pave mcmy patios and walkways
throughout the Muskoka-Parry Sound area.
Bancroft claims (he title "Rock Hound Capital
of the World. " Everx sununer. thousands of
people flock to the town's Geniboree. They
come to .show and trade their rocks, gems and
minerals and comb the surrounding hills and
old mine sites for more.
Mineral specimens which may he found in the
Bancroft area incliule sodalite, apatite,
corundum and many more. These gemstones
and spectacular minerals fonned In the
metamorphic porridge of rocks about one
billion years ago.
Limey sediments collect in warm, shallow
water to form limestone, which becomes
marble when it's metamorphosed.
Marble is common throughout the Central
Metasedimentary Belt. This outcrop from
the Frontenac Terrane Just northwest of
Perth, provides evidence that the terrane
formed in a hot tropical climate.
Glossary
Absolute Age Date
The age of a fossil, rock, or geologic feature, given in years.
Acadian Orogeny
A period of deformadon in the northern Appalachians between 325
million and ,375 million years ago.
Alleghany Orogeny
A period of deformation in the central and southern Appalachians
between 250 million and 325 million years ago.
Amethyst
A variety of quartz that is purple, violet or red. This mineral contains
an excess of iron within its structure. It is used as a gem, and for
decoradve, landscaping, and construction material.
Annelid
A wormlike creature with a segmented body, and distinct head and
appendages, usually known as fossils only from burrows and trails.
Apatite
A fairly common mineral with the composition calcium phosphate.
Human bones and teeth are made of apatite.
Archean
Precambrian rocks more than 2.500 million years old.
Assemblage
A group of rocks of similar age and geologic history.
Basalt
Dark-coloured, fine-grained volcanic rock which is formed when
molten rock cools and hardens rapidly at the Earth's surface. It is high
in iron and magnesium.
Base Metal
Any common metal other than iron, including copper, lead, zinc,
nickel, tin and mercury.
Basin
A depression of the Earth's surface which originated by erosion or
structural changes.
Batholith
A large body of intrusive igneous rock, with a surface exposure of
more than 100 square kilometres, and no known bottom or floor.
Bedrock
Solid rock of the Earth's crust, either exposed at the surface, or
overlain by unconsolidated materials.
Belt
Large-scale geological zone, generally a particular kind of rock
strata exposed at the surface in the form of a zone or band.
Bomb
A piece of volcanic rock larger than 64 millimetres across, that was
blown out of a volcano while fluid, and which received its rounded
shape while in flight.
Brachiopod
A large family of shellfish, with the left side of each shell a mirror
image of the right side of the shell. This is unlike the more common
molluscs, where the top shell is the mirror image of the bottom.
Bryozoan
Tiny, colonial animals which produce skeletons somewhat like those
produced by coral.
Breccia
Rock consisting of angular fragments in a fine-grained matrix.
Bullion
Precious metal such as gold or silver, formed into bars, ingots or other
uncoined form after smeUing and refining.
Calcite
A common mineral and principal constituent of limestone.
Canadian Shield
The main continental block of the Earth's crust underlying North
America. It has been relatively stable over a long period of time and
has undergone only gentle warping. It is composed of Precambrian
rocks.
Cenozoic
An era of geological time from about 66 million years ago to the
present.
Cephalopod
A marine animal with a definite head, and the mouth surrounded by
tentacles. Some varieties have a single "snail" shell, which may be
curved, straight, or coiled. Others, like the octopus, have no shell.
Conglomerate
A rock made up of compacted gravel.
Contact Metamorphism
Alteration of the rocks surrounding an igneous intrusion because of the
effects of heat and the addition of new chemical elements.
Continental Drift
A general term for the theory proposed by Wegener in 1912, thai
continents can move freely across oceanic crust.
Continental Crust
Relatively light-weight rock which floats higher on the soft mantle than
does the oceanic crust. It ranges in thickness from 10 to 70 km, and is
made up mainly of granites and other silica-rich igneous rocks. It also
includes sedimentary and metamorphic rocks.
Continental Shelf
The part of the continental margin between the shoreline and the
continental slope. It has a very gentle slope, and generally extends to
a depth of about 200 metres. From there, the 3" to 6" gradient of the
continental slope runs down lo the deep ocean.
Convection Current
This up-and-down current within the mantle is caused by the intense
heat deep inside the Earth. The heat makes the soft, hot mantle rocks
less dense, causing them to rise. When they rise, they cool, become
heavier and sink back down. This circular flow of mantle rock is
thought to be the main force which moves the tectonic plates.
Core
The spherical region at the centre of the Earth with a diameter of about
6,920 km. It is thought to be composed of a nickel-iron alloy. The
thousands of Jdlometres of overlying rock exert tremendous pressure
on it. Its temperature is estimated to be as much as 4,200°C.
Corundum
An extremely hard mineral composed of aluminum and oxygen. It is
used as an abrasive. Gemmy varieties are ruby and sapphire.
Craton
This is the geological centre of a continent. It marks the area where the
continent first started to evolve. The Canadian Shield is the largest
craton in the world. It covers an area of more than 5.5 million square
kilometres.
Cratonic Basin
A broad depression atop a craton, large enough to hold an inland sea.
Crinoid
A marine animal related to starfish, crinoids are often called "sea lilies"
because they look like flowers rooted on the sea floor.
Cross-bedding
A sedimentary structure in which thin beds of rock are at an angle to the
main bedding planes. Cross-beds are remnants of ripples or dunes
caused by flowing water or wind when the sediments were deposited.
They can be used to determine the flow direction of the wind or currents
at the time of deposition.
Cuesta
An asymmetric ridge with a gentle slope on one side and a steep
slope on the other. The gentle slope is, in many cases, the top of a
bed of sedimentary rock.
Delta
A flat-lopped wedge of sediment formed where a river enters a lake
or ocean. The transporting power of the river water decreases when
it slows as it enters the lake or ocean. The sediments carried by the
river are then deposited to form the delta.
Diabase
A rock with the same chemical composition as basalt and gabbro,
consisting mainly of labradorite (feldspar) and pyroxene.
Dike
A sheet-like igneous intrusion that cuts across the structure of older
rocks. This is spelled "dyke" by some geologists.
Dimension Stone
Building stone that is quarried and prepared in blocks.
Dip
The angle between aflat surface in the rock and a horizontal surface.
Dolostone
Any sedimentary rock consisting essenUally of dolomite, a mineral
very similar to calcite.
Drift
A general term applied to all rock material including clay, sand,
gravel, and boulders transported by a glacier and deposited cither
directly by the ice, or by water running from it.
Drumlin
A low, smoothly rounded oval hill deposited under glacial ice, and
shaped by its flow. Its blunt end points "upstream".
Earthquake
The localized release of energy within the earth's crust. Earthquakes
may be too small to be felt, or may cause severe damage. The
amount of damage depends in part on how deep within the crust the
release of energy takes place.
Elements
These are the basic chemicals that occur naturally in the Earth.
Some of the most common arc oxygen, silicon, aluminum and iron.
Elements combine in different ways to form minerals.
Erosion
A process where earth or rock material is loosened or dissolved, and
then removed from any point of the Earth's surface by water, ice or
wind.
Erratic
A rock carried some distance from its source by glacial ice.
Esker
A long, winding ridge of sand and gravel formed by water flowing
through or under a glacier. Eskcrs run parallel to the direction of glacial
ice flow.
Fault
A fault is a fracture or zone of fractures in the Earth's crust along which
some movement has taken place. Faults are caused by the action of
tectonic plates, and may stretch for many kilometres or may be only a
few centimetres long. One of the most famous is found in California.
It is the San Andreas Fault, which separates the Pacific Ocean plate
from the North American plate.
Feldspar
A group of abundant minerals, feldspars can be cream, pink, grey, or
black. They occur in all types of rocks.
Foreland Basin
A long, nan-ow trough-shaped basin which forms along the edge of a
continent when the continent and another tectonic plate collide.
Gabbro
A coarse-grained, mafic igneous rock which is low in silica and high
in magnesium and calcium.
Glacier
A large ice mass made of recrystallized, compacted snow, which
originates on a land surface and flows under the influence of gravity.
Gneiss
Banded rocks formed during intense regional metamorphism due to the
effects of heat and pressure on pre-existing rocks.
Gondwanaland
An ancient supercontinent in the Southern Hemisphere, which joined
with Laurasia to form Pangaea.
Granite
A coarse-grained, felsic igneous rock consisting of feldspar and quartz
with minor amounts of dark minerals.
Granophyre
A fine-grained granitic rock with intergrown quartz and feldspar.
Greenstone
A geological field term applied to any fine-grained green, volcanic
rock. Its colour is attributed to its high content of the green minerals
chlorite, aclinolite, amphibole and epidote.
Greywacke
A dense grey to greenish grey sedimentary rock composed of sand, sill
and clay.
Group
A general term for a consecutive sequence of related layers of volcanic
or sedimentary rocks.
Hematite
The principal ore of iron, used for centuries as a red pigment,
Hornblende
A common black or dark green mineral.
lapetus Ocean
A sea that existed in the general position of the present Atlantic Ocean
before Europe and Africa collided with North America.
Igneous Rock
Rock formed from the cooling and crystallization of hot, molten
magma at or below the surface of the Earth.
Intrusive Rock
Igneous rock which has forced itself into pre-existing rocks, either
along some definite structural feature, or by deforming and crosscutting the intruded rocks.
Illinoian Stage
The second last major period of glacial advance in North America, it
lasted from about 135,()(}0 to 190.000 years ago.
Iron Formation
A hard, iron-rich sedimcntaiy rock, which is generally Archean in age,
and in most cases contains abundant magnetite.
Ironstone
A hard, iron-rich sedimentary rock, which is generally younger in age
than Archean. and in most cases contains little magnedte.
Island Arc
A curving chain of volcanic islands and earthquake activity commonly
found where an oceanic plate is colliding with another plate.
Isostatic Uplift
Slow upward movement of the surface of the land because a great
weight has been removed, whether by erosion or glacial melting.
Joint
A flat fracture surface in a rock, where no movement has occurred.
This is different than a fault, where movement has taken place.
Kame
A steep-sided, conical hill of sand or gravel deposited by a glacier.
Kettle
A depression formed when a detached block of glacial ice left behind
in glacial drift melts.
Komatiite
A type of dark volcanic rock with low iron, and high magnesium
contents compared to other volcanic rocks.
Laurasia
An ancient supercontinent in the Northern Hemisphere, which joined
with Gondwanaland to ft)rm Pangaea.
Laurentide Ice Sheet
The great ice sheet that advanced and retreated twice between 19().()()0
and 10.000 years ago. When at its maximum size, it covered all of
Canada east of the Rocky Mountains, as well as the northern part of the
United States.
Lava
Molten magma that reaches the Earth's surface and is extruded by a
volcano. It also refers to the same material in its solidified form, after
cooling.
Limestone
Any sedimentaiy rock consisting essentially of calcite.
Magma
Molten rock which is below the surface of the Earth.
Mantle
The zone between the Earth's crust and its core. It is approximately
2,870 km thick.
Marble
A metamorphic rock made of recrystallized calcite or dolomite, pol­
ished marble has many architectural uses.
Mesa
An isolated, nearly level landmass surrounded by steep slopes or cliffs,
which stands high above the surrounding countryside.
Mesozoie
An era of geologic time, the Mesozoie extended from about 225 to
about 66 million years ago.
Metamorphic Rock
Rock formed by transformation of any type of pre-existing rock, while
it remains in the solid state.
Metamorphism
The process of altering a rock in the solid state as a result of heal,
pressure, stress, and the chemical environment.
Meteorite
Meteors are solid chunks of rock which race through space. Most
disintegrate into dust as they pass through the Earth's atmosphere.
Those that do fall to earth arc called meteorites. Some geologists think
that a meteorite crashed into the Earth around Sudbury about 1.85
billion years ago to make the world-famous Sudbury Structure.
Mica
A group of common minerals that form large, flat mineral grains that
can be easily split into very thin, tough, tlat sheets.
Mid-Ocean Ridge
An active volcanic mountain range that extends along the centre of an
ocean. The Mid-Atlantic Ridge is a mid-ocean ridge which formed
when the Atlantic Ocean grew between the separating continents of
Europe and North America, and is the site of continued spreading in the
Atlantic Ocean.
Midcontinent Rift
A rift valley which split the North American craton about 1.11 billion
years ago. It extends from Minnesota, under Lake Superior to Sault
Ste. Marie, and from there under Michigan to Windsor. It filled with
a 30 kilometre thick pile of volcanic rocks, the thickest known on Earth.
Migmatite
A banded rock made up of a mixture of igneous and metamorphic
rocks. Most migmatites have a "stirred" appearance.
Mineral
Minerals are naturally occurring chemical compounds. There are about
3,500 minerals in the world, but only about 20 or so common ones.
Rocks are made of minerals. Some rocks consist of grains of just one
mineral. Quartzite, for example, contains only quartz. Granite is made
of several minerals including quartz and feldspar.
Moraine
A mound, ridge, or other accumulation of glacial debris left behind by
direct glacial action. Different forms have their own names.
Mudstone
A blocky, massive sedimentary rock formed of hardened mud.
Norite
A dark, coarse-grained igneous intrusive rock similar to gabbro.
Oceanic Crust
A relatively dark, heavy igneous rock, which comprises the Earth's
outer rock shell beneath the oceans. It is only 5-\ 0 kilometres thick.
Oceanic crust is relatively heavy and less buoyant than continental
crust, so floats lower on the mantle than does the continental crust.
Ore Deposit
An ore deposit is any type of mineral deposit which can be mined and
processed at a profit.
Orogeny
This is the Latin word geologists use for the process of mountainbuilding. There have been several orogenies, or mountain building
periods, in Ontario from its earliest history to about 1.00 billion years
ago.
Orthogneiss
The general name for gneiss formed of metamorphosed igneous rock.
Outwash Plain
A broad, gently sloping sheet of sediment deposited by streams
flowing out from in front of a glacier.
Outcrop
Geologists love outcrops. These are exposures of rocks or mineral
deposits that can be seen on the Earth's surface because they are not
covered by other deposits or water.
Paleozoic
An era of geological time from about 570 to 225 million years ago.
Pangaea
The name given to the huge supercontinent that included all the land
masses that existed on the Earth 300 million years ago.
Phanerozoic
The portion of geological time when evidence of life is abundant, that
is from about 570 million years ago to the present.
Photosynthesis
The process by which plants use the sun's energy to combine water and
carbon dioxide to make carbohydrates and oxygen.
Pillow Basalt
Rock formed when lava oozes from underwater fissures and cools in
the form of distorted, globular masses.
Plate Tectonics
This geological theory says that the Earth's crust is made up of about
10 major plates which are always on the move. This movement is
thought to be the main cause of most geological processes: mountain
building, the formation of continents and oceans, volcanic eruptions,
and earthquakes. The theory even helps explain how mineral and metal
deposits are formed, and why certain types of deposits are found with
certain types of rocks. Ontario scientist, John Tuzo Wilson, was instru­
mental in developing this important theory in the 1960s.
Plutonic Rocks
These are igneous rocks which formed when magma cooled and
hardened beneath the Earth's surface. They are named after Pluto,
Greek god of the underworld.
Precambrian
The period of time from 4,600 million years ago to 570 million years
ago. The name also refers to rocks formed during this time.
Proterozoic
The more recent of the two great subdivisions of the Precambrian.
Pumice
A volcanic rock containing so many gas bubbles it may float.
Pyrite
A widespread mineral found in many rock types, whether igneous,
sedimentary or metamorphic. It usually forms small, brass yellow
cubes and is commonly known as "fool's gold".
Quartz
A mineral composed of silicon dioxide and found in many rock types.
It is the most abundant mineral in continental crustal rocks. Amethyst
is a coloured variety of quartz.
Quartzite
A sedimentary rock made up mostly of sand-sized grains of quartz, or
the rock formed by metamorphosing the sedimentary rock.
Quaternary
The period of time from about 1.5 million years ago to the present.
Refinery
A place where various techniques are used to remove impurities from
metals. Refining is the final stage in the production of metals.
Regional Metamorphism
A general term used for metamorphism. generally as a result of heat and
pressure which affects a large area.
Relative Age Date
The age of a fossil, rock, or geologic feature, whether older or younger,
as compared to other fossils, rocks, or geological features.
Rhyohte
A common type of light-coloured volcanic rock.
Rhythmite
A unit of silt and mud or siltsone and mudstone, with alternating
coarser and finer grained layers. Rhythmites form in glacial lakes, with
each pair ol" layers representing one year's deposits.
Rift
A rift occurs where the crust has been stretched and fractured, forming
an immense crack. Hot magma from the upper mantle pushes up into
the region. If the crust continues to stretch, the crustal plate splits into
two separate plates, and a new ocean develops between them.
Topographically, a rift is marked by an elongated valley with steep
parallel sides.
Rift Valley
These are valleys which mark spots where continents tried to break
apart, but haven't had enough time to split up, or didn't succeed in
splitting up. There is ahuge rift valley in Ontario called the Midcontinent
Rift. It formed 1.11 billion years ago when the North American
continent tried to split. The valley is along a line centred beneath Lake
Superior and stretches under the state of Michigan to the Windsor area.
Roche Moutonnce
A small knob of bedrock, sculpted by flowing ice to have a gentle slope
on the side from which the ice advanced, and a steep, rubbly drop-off
on the other side.
Rock Cycle
This is the never-ending rock-making process. Here's how it works.
Hot magma from inside the Earth rises to the surface. In some
places, the magma cools and forms igneous rocks within the Earth's
crust.
Rock Cycle - cont'd
Before the fiery liot magma cools, however, it bakes the surrounding
rocks and changes them into metamorphic rock. In other places,
volcanoes erupt and spew out lava which cools into volcanic rock.
Rocks on the Earth's surface are broken up and worn down by the wind,
rain and ice. The bits and pieces of rock are carried back to the sea by
wind, rain and rivers. These fragments collect as sediment on the bed
of the sea and over rime are cemented together into sedimentary rocks.
Layers of sedimentary rock may eventually reach the surface as sea
levels change or if two plates collide and force the rocks up into
mountains.
The heat and pressure generated by the collision of two plates may be
so great that sedimentary and igneous rocks caught between them are
changed into metamorphic rocks.
When an oceanic plate crashes, it can be pulled down into the mantle,
where it starts to melt into new magma. The cycle then begins again.
Sandstone
A sedimentary rock made up mostly of sand-sized grains of quartz, or
the rock formed by metamorphosing die sedimentary rock.
Sangamonian Interglacial
The Sangamonian Interglacial Stage, from about 13.5,000 years to
I 15,000 years ago. was a period between glacial advances in Ontario
when the climate was as warm as, or warmer than, it is today.
Schist
A metamorphic rock that can be easily split into thin slabs, due to the
parallel orientation of the abundant flat mineral grains in it.
Sedimentary Rock
Rock that formed by the accumularion of sediment in water or air. The
sediment may consist of rock fragments, parades of various sizes, or
the remains or animals or plants.
Shale
A sedimentary rock which is formed from mud and clay, and which
breaks easily into Hat sheets.
Shatter Cones
Conical fragments of rock from a few centimetres to more than a metre
high, with disrinctive longitudinal striations. Formed by very highenergy shock waves, they are thought to indicate sites of meteorite
impact.
Sill
An intrusion forced between layers of pre-existing rock, roughly
parallel to these layers, like the filling of a sandwich.
Siltstone
A compact, blocky sedimentai^ rock composed inainly of silt.
Smelter
A place where metal-bearing minerals are heated until they melt and
the sulphur they contain burns off The remaining material is mixed
with quartz, which reacts with the mixture, leaving molten slag and
impure metal. The slag then forms one layer, and the impure metal
another, allowing them to be sepiuated.
Sodalite
An attractive blue mineral used as a semi-precious gem.
Striation
One of multiple scratches, generally parallel, made on the surface of the
bedrock by rocks dragged along by an advancing glacier.
Strike
The direction pointed out by the line where a flat surface in a rock
intersects with an imaginary horizontal surface.
Stromatolite
A layered sedimentary structure formed when bits of sediment are
trapped by colonies of micro-organisms such as algae. They form
some of the world's oldest fossils, and are still being formed today.
Stromatoporoid
A general name for an extinct group of marine animals, possibly related
to sponges, which secreted calcarous skeletons.
Subduction
When a continental plate collides with an oceanic plate, the dense
oceanic plate is usually forced under the lighter continental plate in a
process called subduction.
Sudbury Basin
A huge, oval-shaped bowl 27 by 60 kilometres across, filled with
Precambrian sedimentary rocks. For many years, it was thought to be
the site of an immense volcanic explosion, but new evidence suggests
that it may be the remnant of a meteorite impact crater.
Sudbury Igneous Complex
A thick bowl-shaped layer of igneous rocks of widely debated origin.
The world-famous Sudbury copper-nickel deposits arc located at the
bottom of this layer, and in offshoots from it. Cobalt, gold, silver,
platinum, palladium, and several other metals arc also recovered from
Sudbury ores.
Sudbury Structure
The unique geological structure made up of the Sudbury Basin and the
Sudbury Igneous Complex. The rocks of these two features are not
found anywhere outside the Sudbury Structure.
Supergroup
A formal name used to link related formations or groups of rock.
Suspect Terrane
This is a block of land which has been carried along the Earth's plate
tectonic conveyor belt. The Parry Sound Terrane in the heart of
Ontario's cottage country is a suspect terrane. Geologists think it was
a part of an ancient volcanic island arc which formed thousands of
kilometres away from Ontario - but no-one knows where.
Taconic Orogeny
A period of deformation in the Appalachian region between 460
million and 440 million years ago.
Talus
Pieces of rock of all shapes and sizes which have eroded from cliffs or
steep hills, and fallen in a pile of rubble at the base of the cliff
Terranes
These are sections of land, such as island arcs, which have been swept
against the edge of eratons lo enlarge conlinents. Geologists study
terranes to discover the history of continents.
These sheets of jumbled geological debris such as clay, silt, sand,
pebbles, cobbbles. and boulders are deposited by glaciers.
Trilobite
These marine creatures had three-part exoskeletons consisting of a
head, thorax, and tail. Some modern insects resemble these bottomdwellers, which Uved between 570 and 225 million years ago.
Turbidity Current
Turbidity currents form when piles of sediment on the continental shelf
overbalance, mix with water, and rush down the continental slope at
speeds of up to 60 kilometres per hour.
Unconformity
This is a major break in the sequence of rock layers within sedimentary
rocks. It separates a younger layer of rock from older rocks, and usually
represents a period of erosion.
Volcano
A vent or fissure in the Earth's crust through which molten magma, hot
gases and other fluids escape to the surface of the land, or in some
cases, the bottom of the sea.
Volcanic Eruptions
As oceanic plates are dragged under other plates during plate colli­
sions, the high temperatures and pressures involved generate magma.
Because the magma is less dense than the overlying plate, the magma
rises slowly through the crust. If and when it reaches the surface, it
pours out as lava from a volcanic eruption.
Weathering
The decomposition of rock due to the forces of nature, leading to the
formation of soil.
Wisconsinan Stage
The last major period of glacial advance in North America, it lasted
from about 10,000 years ago to about 115,000 years ago. During this
period, there were a number of glacial advances and retreats. Parts of
Ontario were ice-free during the times of glacial retreat.
Zircon
This mineral is made up of zirconium, silicon and oxygen. It is one of
the minerals isolated for testing during absolute age determinations,
because many grains of zircon contain minute amounts of radioactive
elements as impurities. Some varieties of zircon are used as gemstones.